1. Field of the Invention
The present invention relates to an electro-thermal printing apparatus, and more particularly, to a printing head for a resistive ribbon type printing apparatus. The ribbon used in such a printing apparatus comprises a flexible base insulating (or conductive) film, a layer of thermal transferable ink facing a paper, and an electrically resistive layer facing the printing head. When an electric current is made to flow through a portion of the resistive layer from the printing electrodes coming into contact with the resistive layer, Joule heat is generated at the portion at which the current is flowing, to melt a portion of the ink layer, and the molten ink is transferred onto a paper to form a printed image.
2. Description of the Related Art
Such a resistive ribbon type printing apparatus and the printing head thereof have been proposed in, e.g., U.S. Pat. Nos. 3744611, 4350449, and 4456915. Also such printing heads are disclosed in, e.g., Japanese Unexamined Patent Publication (JUPP) Nos. 60-214972, 60-214971, 61-295050, and 61-295051.
According to JUPP No. 60-214972, a printing head is produced by printing a conductive paste including a hard metal such as W, Mo and Mn over the entire surface of a ceramic green sheet, sintering the printed green sheet, and selectively etching the sintered metal layer (metallized layer) to form a plurality of electrodes, by a photolithography process. In this case, for example, the printing head comprises a ceramic substrate made of magnesia and silicon dioxide and having a hardness of from to 500 to 600 Hv, and tungsten electrodes having a hardness of about 700 Hv and a density of 3 electrodes per mm. When the printing head is operated, the substrate and electrodes come into contact with the ribbon simultaneously. Nevertheless, a conventional ceramic substrate has a relatively high hardness, and therefore, the electrodes should have a higher hardness, which leads to the problem of an insufficient formation of fine pattern electrodes, as it is difficult to selectively etch the hard metal (W) layer to form fine electrodes.
According to JUPP No. 60-214971, a printing head is produced by depositing a conductive layer over the entire surface of a ceramic substrate by a vacuum evaporation or sputtering process, forming a plating layer on the conductive layer by an electroless plating process, and selectively etching the layers to form a plurality of electrodes by a photolithography process. In this case, for example, the ceramic substrate is made of magnesia and silicon dioxide and has a hardness of from 500 to 600 Hv, and the electrodes are made of Ni-W plating layer having a thickness of 10 .mu.m and a hardness of about 800 Hv. Since it is difficult to selectively etch such a hard alloy plating layer to form fine pattern electrodes, the obtained electrodes have an electrode density of 3 lines/mm. Furthermore, an adhesion strength of the electrodes (i.e., the conductive layer of Ni-W) to the substrate is low, since the vacuum evaporation (or sputtering) process is adopted.
According to JUPP No. 61-295050, a narrow stripe insulating projection is formed on a ceramic substrate, and a plating layer is formed on the projection and the substrate and selectively etched to form electrodes. In operation, only the plating layer portions covering the projection come into contact with the ribbon, i.e., the substrate does not come into contact with the ribbon.
According to JUPP No. 61-295051, a plating layer is formed on a ceramic substrate and is selectively etched to form electrode lines, electrode contacts are formed by build-up plating on predetermined portions of the electrode lines, and only the electrode contacts come into contact with the ribbon.
In both JUPP No. 61-295050 and JUPP No. 61-295051, the service life of the printing heads is limited by the wear of the contact portions of the electrodes, and since the thickness of the plating layer at these contact portions is relatively thin (for example, 10 to 40 .mu.m), the printing heads have a relatively short service life.